1. Field of the Invention
This invention relates to an apparatus for interferometrically measuring the thickness of film or evaporated film or the glass-to-glass interval and generally, optical phase such as thickness or refractive index of optically transparent substance film.
2. Description of the Prior Art
An apparatus for interferometrically measuring optical phase is disclosed in U.S. Pat. No. 2,518,647 issued on Aug. 15, 1950. This U.S. Patent shows a technique wherein the wave fronts coming from both sides of a film illuminated by a source of white light are respectively directed to an interferometer comprising a tilted translucent surface and an untilted reflecting surface, so that each of the two wave fronts from the film is amplitude-split by the tilted translucent surface and the other two wave fronts split by the reflection are tilted and directed to an interference surface, while the two wave fronts split by the transmission are reflected by the untilted reflecting surface and directed to the interference surface, whereby white interference patterns are formed on the interference surface. Then the film thickness is measured from the difference in length of optical path between the positions whereat the mutually intensifying interference patterns are formed. Similar methods of measurement are known from U.S. Pat. Nos. 2,578,859 and 2,655,073. However, all these are of the type in which the measurement of film thickness is effected from the difference in length of optical path between the positions whereat the mutually intensifying interference patterns are formed, and this is inconvenient to the automatization of the measurement. An apparatus which has overcome such inconvenience is disclosed in U.S. Pat. No. 3,319,515 (corresponding German Pat. No. 1,447,253).
According to this apparatus, the light beams from a first and a second surface of a specimen illuminated by a source of broadband light are directed to an interferometer such as Michelson's interferometer or the like and one of two split optical paths of the interferometer is vibrated over a certain span, namely, one of two mirrors to which the two wave fronts split by the beam splitter of the Michelson's interferometer are respectively directed is vibrated over a certain span so as to produce an optical path difference with respect to the other mirror; the amount of displacement during such vibration is represented by time axis, and the lapse of time between a time point whereat the interference peak resulting from the vibration appears and a time point in said span which corresponds to the position whereat the vibrating mirror lies is measured and the optical phase is measured with the measured lapse of time as the function. A feature of this apparatus is that it measures the lapse of time and therefore, the vibrating mirror must be linear with respect to time. Such a mirror which vibrates linearly with respect to time is extremely difficult to obtain. This is apparent from the fact that the inventor of such method shows in his patent specification the method of measuring the pulse number corresponding to the amount of displacement of the vibrating mirror, instead of measuring the lapse of time.